A key tracer used to estimate how much atmosphere Mars lost can change depending on the time of day and the surface temperature on the Red Planet, according to new observations by NASA-funded scientists. Previous measurements of this tracer—isotopes of oxygen—have disagreed significantly. An accurate measurement of this tracer is important to estimate how much atmosphere Mars once had before it was lost, which reveals whether Mars could have been habitable and what the conditions might have been like.

Mars is a cold, inhospitable desert today, but features like dry riverbeds and minerals that only form with liquid water indicate that long ago it had a thick atmosphere that retained enough heat for liquid water—a necessary ingredient for life—to flow on the surface. It appears that Mars lost much of its atmosphere over billions of years, transforming its climate from one that might have supported life into the desiccated and frozen environment of today, according to results from NASA missions such as MAVEN and Curiosity and going back to the Viking missions of 1976.

However, many mysteries about the Red Planet's ancient atmosphere remain. "We know Mars had more atmosphere. We know it had flowing water. We do not have a good estimate for the conditions apart from that—how Earthlike was the Mars environment? For how long?" said Timothy Livengood of the University of Maryland, College Park and NASA's Goddard Space Flight Center in Greenbelt, Maryland. Livengood is lead author of a paper on this research published online in Icarus August 1.

One way to estimate how thick the original atmosphere of Mars was is to look at isotopes of oxygen. Isotopes are versions of an element with different mass due to the number of neutrons in the atomic nucleus. Lighter isotopes escape to space faster than heavier isotopes, so the atmosphere that remains on the planet gets gradually enriched in the heavier isotope. In this case, Mars is enriched compared to Earth in the heavier isotope of oxygen, 18O, versus the lighter and much more common 16O. The measured relative amount of each isotope can be used to estimate how much more atmosphere there was on ancient Mars, in combination with an estimate for how much faster the lighter 16O escapes, and assuming that the relative amount of each isotope on Earth and Mars was once similar.

The problem is that measurements of the amount of 18O compared to 16O on Mars, the 18O/16O ratio, have not been consistent. Different missions measured different ratios, which results in different understandings of the ancient Martian atmosphere. The new result provides a possible way to resolve this discrepancy by showing that the ratio can change during the Martian day. "Previous measurements on Mars or from Earth have obtained a variety of different values for the isotope ratio," said Livengood. "Ours are the first measurements to use a single method in a way that shows the ratio actually varying within a single day, rather than comparisons between independent devices. In our measurements, the isotope ratio varies from being about 9% depleted in heavy isotopes at noon on Mars to being about 8% enriched in heavy isotopes by about 1:30pm compared to the isotope ratios that are normal for Earth oxygen."

This range of isotope ratios is consistent with the other reported measurements. "Our measurements suggest that the previous work all may have been done correctly but disagreed because this aspect of the atmosphere is more complex than we had realized," said Livengood. "Depending where on Mars the measurement was made, and what time of day on Mars, it is possible to get different values."

The team thinks the change in ratios during the day is a routine occurrence due to ground temperature, in which the isotopically heavier molecules would stick to cold surface grains at night more than the lighter isotopes, then are freed (thermally desorb) as the surface warms up during the day.

Since the Martian atmosphere is mostly carbon dioxide (CO2), what the team actually observed were oxygen isotopes attached to carbon atoms in the CO2 molecule. They made their observations of the Martian atmosphere with the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, using the Heterodyne Instrument for Planetary Winds and Composition developed at NASA Goddard. "While trying to understand the broad spread in estimated isotope ratios that we retrieved from the observations, we noticed that they were correlated with the surface temperature that we also obtained," said Livengood. "That was the insight that set us on this path."

The new work will help researchers refine their estimates of the ancient Martian atmosphere. Because the measurements can now be understood to be consistent with the results of such processes in other planets' atmospheres, it means they are on the right track for understanding how the Martian climate changed. "It shows that the atmospheric loss was by processes that we more or less understand," said Livengood. "Critical details remain to be worked out, but it means that we don't need to invoke exotic processes that could have resulted in removing CO2 without changing the isotope ratios, or changing just some ratios in other elements."

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New insight into how much atmosphere Mars lost (2019, September 6)
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User comments

Most discussions of Martian atmosphere losses center around erosion caused by the solar wind. Yet, the earth is much closer to the sun than Mars, so it must have lost a lot as well. I realize that earth has a deeper gravity well so there is definitely a tradeoff, but I haven't seen a lot of studies that show it. The moon was also a lot closer to the earth in its early history, so there must have been a substantial effect from that.

At any rate, it seems to me that projecting a much different atmosphere density for eath early in the solar system's history is reasonable. Where are the studys?

"It shows that the atmospheric loss was by processes that we more or less understand,"

no , it doesn't. i love planetary science, but scientists are always emphasizing how much more certain their results are , than they are. that is a fact. google the reproducibility crisis and the many scientisits who simply say that the p values should be another order of magnitutde ( 4 deviations not 3) in this instance, we are being an entire complex system which barely now exists has been deconstructed from scant past geological history tracing analysis. i don't buy it one bit. they're are doing great science , but there is MUCH more to do and we haven't really even done anything but scrap the surface of mars. the sample size is WAY to small. ridiculous and potentially insufficient. it's irresponsible to say 'we've figured out the history" of the entire atmosphere. ridiculous

No you don't. All the features they ASSUME were caused by flowing water are much more likely to have been caused by electromagnetic means, as are all the ones on Earth that they again ASSUME were from water action. As usual, mainstream has everything backwards.

Regarding the speculations about the timeline, current assumptions are very off. The loss of atmosphere occurred as recently as 75,000 years ago. Here the details, para 21 onward:http://www.future...port_201

Most discussions of Martian atmosphere losses center around erosion caused by the solar wind. Yet, the earth is much closer to the sun than Mars, so it must have lost a lot as well. I realize that earth has a deeper gravity well so there is definitely a tradeoff, but I haven't seen a lot of studies that show it. The moon was also a lot closer to the earth in its early history, so there must have been a substantial effect from that.

The gravity well effect is a good prior, and the hypothesis of the so called planetary shoreline (an effect of Jeans escape from such wells) show that Mars would have little atmosphere anyway. The difference between gravitational wells show up in Venus vs Earth, where the former lost its hydrogen by UV dissociation and solar wind attrition in the higher atmosphere. Moon's early magnetic field apparently did limited damage.

The martian solar wind erosion is well established by MAVEN, and it seems that the non-dipole field of Mars rather hurts.

"It shows that the atmospheric loss was by processes that we more or less understand,"

no , it doesn't.

Yes, it does - these are the experts and your personality incredulity means nothing. You refer to the reproducibility problem in psychology, but this is physics. Your Dunning-Kruger trait is really showing.

This astute observationIs indicativeAs to this Martian atmosphere constituents billions of years agoFore If we take Earth's atmosphere on EarthIf Earth loses its atmosphereSo that its density match's marsWhat percentage of this remaining atmosphere

This astute observationIs indicativeAs to this Martian atmosphere constituents billions of years agoFore If we take Earth's atmosphere on EarthIf Earth loses its atmosphereSo that its density match's marsWhat percentage of this remaining atmosphere

Will be carbon dioxide (CO2)

CO2 is much heavier than O2, it would be be the predominant gas like on mars and venus

This astute observationIs indicativeAs to this Martian atmosphere constituents billions of years agoFore If we take Earth's atmosphere on EarthIf Earth loses its atmosphereSo that its density match's marsWhat percentage of this remaining atmosphereWill be carbon dioxide (CO2)

peabody3000> > CO2 is much heavier than O2, it would be the predominant gas like on mars and venus

If Earth loses its atmosphereSo that its density match's MarsWhat percentage of this remaining atmosphere, will be carbon dioxide (CO2)

Earth's atmosphere will be identicalto the constituents of this martian atmosphereas is today!

as we exist on this earthly earthas we live on this earthas we live and breathe on this earth

in this earthly atmospherethis atmosphere that like marshas existed as long as mars

existed as long as marsfore this earthly planet and planet marscame into being simultaneously

where as this earthly atmospheregrew flourished gave life to these livingwhere these living nourished this earthly atmosphere

this martian atmospherein its infancy mirrored this earthly atmospherethis martian planet mars lost its tenuous grip on its atmosphere

for is its possible to terraform this earthly atmospherewhen this earthly earth's atmosphere mirrors this martian atmosphere as it is todayis it possible to terraform this earthly atmosphere when it only contains 95.32% carbon dioxide

Every planet has escape velocity depending on its mass. Every gas has molecular speed distribution depending on temperature. Build some kind of chart "temperature-escape velocity". Then place planets there. You can do it yourself or take it from https://en.wikipe...ape.png.So: 1) this chart shows why Mars never had oceans/seas/rivers in earth meaning; 2) this chart shows that despite Mars is in "oxygen-nitrogen" zone, any impact moves it in "carbone dioxide" zone. This is why Mars has such atmosphere which it has.

Welcome to Physorg, @Mikhail G_I hope you enjoy your stay. Don't mind the trolls such as rrwillsj and some others who aren't really interested in science, but only to annoy commentators and take up valuable space.

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